Electric high pressure discharge lamps



Filed Jan. 21, 1953 IN V EN TOR.

M7494? fay 2 M 2,784,347 ELECTRIC HIGH PRESSURE DISCHARGE LAMPS WolfgangEmery Thouret, East Orange, N. J., "assignr to Westinghouse ElectricCorporation, East Pittsburgh, Pin, a corporation of PennsylvaniaApplication January 21, 1953, Serial No. 332,481

1 Claim. (Cl. 315-47) My invention relates to electric discharge lampswith solid incandescent electrodes, the electrode leads of which areconected by incandescent wires or incandescent coiled filaments parallelto the discharge path for the purpose of starting the discharge. Usuallythe wires or wire coils have such dimensions that they glow brightlywhen the mains voltage is switched on. They ionize the discharge path byemitted electrons and thus bring about or facilitate starting of thedischarge. This starting arrangement has considerable fundamentalimportance as, other. than contact starting by means of mechanicallymovable electrodes, it is the only arrangement that renders possibleinstantaneous starting of high pressure lamps in any state of operationat standard mains voltageswithout auxiliary electrodes and withoutadditional external electric devices. However, up to the present itcould be applied inpractice only to low pressure discharge lamps andonly to such types at very low voltages under 30 volts.

For perfect and reliable operation of the arrangement the starting wireor starting filament must have such dimensions that its resistance perunit length is at least equal to or greater than that of the dischargepath after starting. As a consequence of this condition, the startingfilament must be made of a rather thin wire that burns throughinstantaneously if the arc discharge starts from it even only for ashort moment. The danger of burning through is considerably greater inhigh pressure lamps than in low pressure lamps because the high pressureare forms a concentrated hot spot and has a much higher energy densityin its starting point.

Several difierent means have been suggested to avoid that an arestarting point is formed and stays at the starting filament because oneisaware of the fact that this can result in decreasing its life oreventually in burning it through quickly also in low pressure lamps.These measures e. g. consist in having the starting coil pass closely tothe main electrodes so that they are preheated by the glowing filamentor the are already started at it. Thus the arc shall be influenced tojump over from the filament to the main electrode. According to anothersuggestion the starting coil is :conencted to the electrode leads nottightly but only loosely so that undefined contact resistances areformed that can favor the jumping of the arc from the filament to themain electrodes. All these suggested arrangements have been foundunreliable for use in high pressure lamps because they are based on thestart of the are at the filament and only aim to have the arc jump fromthe filament to the main electrodes later. It has been proven byextendedexperiments that in practice only such starting arrangements canbe used inhigh pressure lamps that principally avoid starting of the areat the filament. Thus no'part of the starting filament acts as electrodeduring the starting procedure. The filament may be used exclusively as ameans to create thermal electrons for the purpose of ionizing the maindischarge path. It is the purpose of my invention torealize the abovementioned principle by several new arrangements or measures that can beused alone or simultaneously. Thus the known starting arrangement withincandescent parallel filament canbe improved so that italsoworksreliably in high pressure lamps of all kinds operating with solidincandescent electrodes at op e. g. as Wire coils arranged in the spacesbehind the main; electrodes. But as there is very limited space behindthe.-

crating pressures higher than 0.5 atm. and at voltages higher than v. t

' According to my invention the starting filament is not connecteddirectly at both ends to the main electrode leads but: only throughcertain solid resistors arranged inside or outside the discharge tube.As soon as a current.

flows through the starting filament after the mains volt-.

age has been switched on, the voltage drops in the re-= sist ors resultin the field strength along the filament being lower than that betweenthe main electrodes before the arc has started. Thus the main dischargestarts directly at the main electrodes and no intermediate are startingthe voltage between the filament ends to less than one;

'half or one third of the voltage on the main electrodes before startingof the arc. But in any case the resistors in series to the filament haveto be so great that in each of them occurs 'a minimum voltage drop of atleast 10% of the mains voltage or transformer voltage with which thelamp is operated. V v The resistors can be manufactured by any method,

electrodes usually they will be made preferably of refractory materialswith high resistivity as e. g. semi conducting refractory metalcompounds. In the case of limited space within the discharge tube theends ofthe starting filament can be led outside by special seals'and canbe connected outside through the resistors to the leads of the mainelectrodes. In this case the resistors will be in the bases or withinthe outer bulbs of the lamps preferably. 7 7

According to my invention another measure for pre venting an arcstarting spot'on the filament or for trans; ferring it safely andinstantaneously to the main electrodes is the following: The electrodeshave deep grooves or off-center holes parallel to the discharge axis andparallelto the starting filaments through which. the ends of thestarting filament are led. .The starting filaments are fastened in suchway that they are enclosedpartlyor completely by the electrode bodiesbut dont touch the electrodes. The starting filaments are electrically,.a"nd' mechanically connected only at the rear electrode endsor at theelectrode leads, eventually through the above meni tioned seriesresistors. In case the filaments are not stiff enough to avoid safely acontact to the-electrodes, when the lamp is shaken, they can be equippedwith a core fof refractory insulating or poorly conducting material.Such cores can consist e. g. of sintered purealu'minum oxide, zirconiumoxide, or thorium oxide. Finally according to my invention the'startingfilaments are switched off the main electrodes immediately afterthe'main arc has started for protecting them against burning through andsecuring the immediate transfer ,.'6f the arc. For this purpose usuallythe ends of the" starting filament must be led outside of the dischargetube'T-by means of separate seals. As switches preferably" on 1' twoglow discharge bimetal-relays, well known as' 'f g starters, are used.In principle any kind of relay fiuenced by the lamp voltage or anymanually spent switch can be used. In switching the starting filam ntsoif the electrodes the additional advantage is won in they consume nowattage during lamp operationjand' consequently the operating efiiciency"of 'th'e'la'nipT is injured by the filaments. QThe startingarrangements imprjo've H Patented Mar. 1951.

operating pressures from 0.5 to or very high pressures up to 100 atm. ormore. They can have activated or purely metallic solid incandescentelectrodes and they can be filled with pure metal vapor, pure rare gas,or any combined mixture of metalvapors and rare gases.

The starting arrangement can consist of one or more starting filamentsparallel to the discharge path. It can also be applied to lamps withmore than one pair of electrodes or more than one are.

'The accompanying drawing shows two practical examples of high pressurelamps with starting arrangements according to my invention. Fig. 1represents a spherical super high pressure lamp with rare gas filling ofabout 20 atm. suitable for operation on 110 v. D. C. or A. C. mainsvoltage. Fig. 2 represents a tubular high pressure mercury vapor lamp.In the Fig. 1 lamp, the rating is 2000 watt at an arc voltage of about35 v. and an operating current of about 60 amps. Both relatively largeelectrodes E consist of pure tungsten and have off-center holes Lparallel to their axis through which the ends of the starting filament Zextend freely without touching. The filament contains a core ofs'intered thorium oxide rod to attain the stiffness necessary to avoidelectrical contact to the electrode. Behind the electrodes E thestarting filament Z is connected to the electrode leads through bothseries resistors R1 and R2. R1 and R2 are 3 ohms each, the filamentitself has a resistance of about 16 ohms. Thus, when the lamp isswitched on, the current in the starting circuit is about 5 amps. andbetween the electrodes and the starting filament ends a voltage drop of15 v. occurs. Thus the field strength between the front surfaces of themain electrodes is considerably greater than that between any parts ofthe starting filament. Besides this the resistance per unit length ofthe starting filament is considerably greater than that of the are afterstarting. When the arc is started the voltage between the mainelectrodes drops to about 35 v., the current in the starting circuitdecreases to about 1.5 amps. the latter being negligible compared to thearc current of about 60 amps.

Fig. 2 shows a mercury high pressure lamp of about 3 atm. operatingpressure with tubular quartz bulb. It has 300 watts at about 120 v. arevoltage and 2.6 amps. operating current. The electrodes B have deepgrooves N offset from the eelctrode axis and parallel thereto. Throughthese the starting filament is led freely without touching. The ends ofthe starting filament vare led outside by means of separately sealedleads for the purpose of switching olf after the starting of the maindischarge. Outside of the discharge tube the two resistors R1 and R2 arein series to the starting filament, each having about 30 ohms.resistance. The outer lamp circuit contains the choke-ballast D. Themains voltage Ne is 220 v. The starting filament has a resistance of 160ohms.

After switching the mains voltage on, a current of about 1 amp. flows inthe starting circuit and both ends of the starting filament have apotential difference of about 30 v. each with respect to theadjoiuingmain electrode. When the .arc is formed the arc voltage orelectrode voltage drops to about 120 v. and this decrease is used forswitching off the starting circuit on both ends by means of both glowdischarges bimetal relays (glowstarter) G.

'Theseglow-starters are of special three-electrode design and work asfollows: When the mains Ne is switched on, the full mains voltage isbetween the outer lamp con- Emotions and between the movable bimetalelectrodes '1 and the fixed counter electrodes 3 of the glow-starters.For this purpose the latter are connected to the opposite lampconnections by means of the relatively great auxiliary resistors H. Aglow discharge is started in the glow starters, the current of which islimited by the resistors H. This current heats the Lbimetal electrodes1, the same approachthe electrodes 2 and make contact to them. Bycontact between :1 and '2 the "starting circuit is switched on, thefilament Z starts glowing and the are between the electrodes 5 resultsin a great drop of the electrode voltage, the glow discharge in theglow-starters cannot be maintained and extinguishes. The bimetalelectrodes 1 open the contacts between 2 and 1, thus switching ofi thestarting circuit on both ends. It is switched on again only after themains voltage is connected again to the main electrodes after the erahas b en xt ngu sh The elec rod s in the glowstarters can be arranged sothat the bimetal contact touches both electrodes 2 and 3 at the sametime or shortly one after the other (first 2, then 3). The startingcircuit is closed then only for the short moment between theextinguishing of the short-circuited glow discharge and the moment whenthe bimetal electrodes have cooled down enough to open the contactagain. In this case the glow-starters operate automatically severaltimes until an arc has started between the main electrodes of the lamps.If the starting circuit is to be switched off only on one end of thefilament, this can be done by means of a simple .twoeelect-rodeglow-starter.

Having thus disclosed my invention what I claim as new and desire tosecure by Letters Patent of the United States is:

The combination which comprises an electric highpressure gas-dischargelamp having an envelope with lamp electrodes spaced apart from oneanother within said envelope and adaptedto have an arc dischargemaintained in a path therebetween, and a starting arrangementcomprising, a double-ended starting conductor within said envelope andparalleling and adjacent to said discharge path, first and secondthree-element glow starters positioned exteriorly of said envelope, eachof said starters having a bi-rnetal first electrode, a second electrodeand a third electrode, said starter electrodes being normally spacedfrom one another, a first resistor electrically connected between saidbi-metal electrode of said first starter and said third electrode ofsaid second starter, said til-metal electrode of said first starterbeing directly electrically connected to one of said lamp electrodes, asecond resistor electrically connected between said bi-rnetal electrodeof said second starter and said third electrode of said first starter,said bi-metal electrade of said second starter being directlyelectrically connected to the other of said lamp electrodes, a thirdresistor electrically connected between said second electrode of saidfirst starter and one end of said starting conductor, a fourth resistorelectrically connected between said second electrode of said secondstarter and the other end of said starting conductor, each of saidstarters being adapted to maintain a glow discharge between their firstand third electrodes upon application of an operating potentialthereacross, each of said himetal electrodes upon receiving heat fromsaid glow discharges being adapted to move into contacting relationshipwith the said second electrode of its respective starter to place apotential across said starting conductor to cause same to heat, thebreakdown potential across said lamp electrodes being lowered by theheat of said starting conductor to cause said are discharge to occurthereacross, said are discharge being of sufficiently low resistance toextinguish the glow discharges in said starters to cause each of saidbi-metal electrodes to cool, each of said bi-metal electrodes uponcooling being adapted to resume their normal spaced relationship,whereby said starting conductor is electrically isolated by saidstarters. I

References Cited in the file of this patent UNITED STATES PATENTS1,990,170 Dellian 'Feb. 5, 1935 2,177,105 Gustin Oct. 24 1939 2,263,171Hays V Nov. 118, 1941 12,275,768 "Kern Mar. 10, 194 2

